asset guide, match application.rb snippets to the generated file [ci skip]

`if defined?(Bundler)` is no longer used in our current application.rb
Eliminate minitest warnings

https://github.com/seattlerb/minitest/commit/9a57c520ceac76abfe6105866f8548a94eb357b6#L15R8
Extract AR::Core#inspect and AR::Core.inspect's test code from base_test.rb

The methods got moved to core.rb in commit
b2c9ce341a1c907041f55461aefebb0321280cb5, but the tests never did.
Extract AR::AttributeMethods#attribute_for_inspect's test code out from base_test.rb

The method itself got extracted out from ActiveRecored::Base in commit
ceb33f84933639d3b61aac62e5e71fd087ab65ed, but the test code never did.
Extract AR::Persistence#becomes's test code out from base_test.rb

The method got extracted out from AR::Base in commit
d916c62cfc7c59ab6411407a05b946d3dd7535e9, but the tests never did.
Add ActiveModel requirement to application.rb

Currently, ActiveModel is only loaded by ActiveRecord. If you skip ActiveRecord, ActiveModel will not be required (or even autoloaded) and including `ActiveModel::Model` into a plain Ruby class will raise `NameError`.

To reproduce this:

- create a new app with `rails new my_app -O`
- create a Ruby class that includes `ActiveModel::Model` in `app/models`
- load up a Rails console and try to do anything with the class :-)

Since ActionPack relies so heavily on the ActiveModel API, this should probably be considered a dependency of the app. Another possibility would be to make it a dependency of ActionController.
Merge pull request #10722 from jgaskins/patch-1

Add ActiveModel requirement to application.rb
Merge pull request #10714 from tkhr/extract_test_from_base_test.rb_in_AR

Extract tests code out from AR's base_test.rb
remove code duplication
Merge pull request #10725 from neerajdotname/remove_code_duplication

Remove code duplication
Merge pull request #10667 from rubys/minitest-autorun

Eliminate minitest warnings
Require only minitest.

minitest/autorun required minitest/spec and we are avoiding to require
it.
Add CHANGELOG entry for 99860582b2b1c0fc42bf84c52aac57b243d42678
Fix the `:primary_key` option for `has_many` associations.

When removing records from a `has_many` association it used
the `primary_key` defined on the association.

Our test suite didn't fail because on all occurences of `:primary_key`,
the specified column was available in both tables. This prevented the
code from raising an exception but it still behaved badly.

I added a test-case to prevent regressions that failed with:

```
  1) Error:
HasManyAssociationsTest#test_has_many_assignment_with_custom_primary_key:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: essays.first_name: UPDATE "essays" SET "writer_id" = NULL WHERE "essays"."writer_id" = ? AND "essays"."first_name" IS NULL
```
deprecate Validator#setup (to get rid of a respond_to call). validators do their setup in their constructor now.
Merge branch 'deprecate-validator-setup'

Closes #10716
Merge pull request #10713 from senny/10693_fix_primary_key_option_on_has_many

Fix the `:primary_key` option for `has_many` associations.
Merge pull request #10222 from senny/update_bundler_section_in_assets_guide

docs, adjust the application.rb snippets to match the generated file.

Closes #10449
Add note about decorator loading in Engines guide.

Because decorators aren't referenced by the application, they won't get
autoloaded. And because we recommend the pattern, it would be irresponsible
to not show how to load them properly.

Fixes #10647.
Fix some typo in method names, variables